linux-kernel - Re: [PATCH 19/19] arm64: vdso32: Enable Clang Compilation

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Message-ID: <CAKv+Gu8Qh495twz-3UQrFiKfPq-Kt_o+JrCNwEcdMedV2DqPEA@mail.gmail.com>
Date:   Mon, 17 Feb 2020 23:39:53 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Will Deacon <will@...nel.org>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        0x7f454c46@...il.com, linux-mips@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>, pcc@...gle.com,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        avagin@...nvz.org, Stephen Boyd <sboyd@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        salyzyn@...roid.com, Paul Burton <paul.burton@...s.com>
Subject: Re: [PATCH 19/19] arm64: vdso32: Enable Clang Compilation

On Mon, 17 Feb 2020 at 17:46, Will Deacon <will@...nel.org> wrote:
>
> On Mon, Feb 17, 2020 at 12:26:16PM +0000, Vincenzo Frascino wrote:
> > On 13/02/2020 18:44, Nathan Chancellor wrote:
> > > On Thu, Feb 13, 2020 at 04:16:14PM +0000, Vincenzo Frascino wrote:
> > >> Enable Clang Compilation for the vdso32 library.
> >
> > [...]
> >
> > >> +LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
> > >
> > > Well this is unfortunate :/
> > >
> > > It looks like adding the --target flag to VDSO_LDFLAGS allows
> > > clang to link the vDSO just fine although it does warn that -nostdinc
> > > is unused:
> > >
> > > clang-11: warning: argument unused during compilation: '-nostdinc'
> > > [-Wunused-command-line-argument]
> > >
> >
> > This is why ended up in this "unfortunate" situation :) I wanted to avoid the
> > warning.
> >
> > > It would be nice if the logic of commit fe00e50b2db8 ("ARM: 8858/1:
> > > vdso: use $(LD) instead of $(CC) to link VDSO") could be adopted here
> > > but I get that this Makefile is its own beast :) at the very least, I
> > > think that the --target flag should be added to VDSO_LDFLAGS so that gcc
> > > is not a requirement for this but I am curious if you tried that already
> > > and noticed any issues with it.
> > >
> >
> > --target is my preferred way as well, I can try to play another little bit with
> > the flags and see what I can come up with in the next version.
>
> Yes, please. I'd even prefer the warning rather than silently assuming that
> a cross gcc is kicking around on the path.
>

Doesn't Clang have -Qunused-arguments for that?